A power converter is a device fit for many applications in a power network, used for rectification where electric power flows from an ac (alternating current) side to a dc (direct current) side and for inversion where the power flows from the dc side to the ac side. The power converter may be used in various applications, for example as interconnection between asynchronous systems, for power flow control or for increasing the capacity of existing ac transmission by conversion to dc transmission.
A multilevel converter using a cascade configuration is based on serially connected multiple single-phase cells to each ac phase, wherein the cells comprise semiconductor switches. The multilevel converter having such configuration is in the following denoted cascaded two-level converter, and it can be built using a structure based on such series-connected cells, each cell comprising a capacitor and a pair of semiconductor switches of turn-off type series-connected.
A control device controls the turning off and on of the semiconductor switches in the cells of the cascaded two-level converter, based on measurements of e.g. the voltage on the dc side and a desired reference voltage. The control of the cells in one phase of the cascaded two-level converter is primarily based on the objective to define the switching instants in order to realize a desired fundamental frequency output voltage based on a voltage reference given by an ac current control function.
In order to minimize harmonic interaction between the cascaded two-level converter and the ac-side, which for example could be a three-phase power network grid, it is desirable to define the current control function so as to make the cascaded two-level converter appear as a voltage source behind an inductance. FIG. 1 illustrates an equivalent circuit for the cascaded two-level converter, wherein the equivalent inductance of the cascaded two-level converter equals half of the valve inductance, Lv/2. In the figure, Uv denotes the equivalent ideal voltage source and Iv denotes the corresponding current. Ipcc illustrates the current at the point of common coupling between the dc side and the ac side. Ideally, Ipcc equals Iv.
The capacitors of the cells of the cascaded two-level converter are not infinitely large and a ripple voltage will therefore appear when exposed to the fundamental frequency current in combination with the switching action.
The ripple on the cell capacitors will in turn result in that the output voltage on the ac-side will differ from the voltage reference which means that the equivalent inductance of the cascaded multilevel converter is not equal to Lv/2.